Sealants useful in aerospace and other applications must satisfy demanding mechanical, chemical, and environmental requirements. The sealants can be applied to a variety of surfaces including metal surfaces, primer coatings, intermediate coatings, finished coatings, and aged coatings. In sealants such as those described in U.S. Pat. No. 6,172,179 an amine catalyst is used to provide a cured product. Such systems typically cure in 2 hours to 12 hours and although exhibiting acceptable fuel resistance and thermal resistance for many applications, a longer pot life such as from 24 hours to 72 hours, and improved performance of the cured product is desirable.
Michael addition curing chemistries are often used in acrylic-based polymer systems and, as disclosed in U.S. Pat. No. 3,138,573, have been adapted for use in polysulfide compositions. Application of Michael addition curing chemistries to sulfur-containing polymers not only results in cured sealants having faster cure rates and enhanced performance including fuel resistance and thermal resistance, but also provides a sealant with improved physical properties such as elongation. The use of Michael addition curing chemistries for sulfur-containing polymer compositions useful in aerospace sealant application is disclosed in U.S. application Ser. No. 13/529,237, filed on Jun. 21, 2012, which is incorporated by reference.
The compositions disclosed in U.S. application Ser. No. 13/529,237 employed one or more base catalysts such as amine catalysts. In the presence of a strong base such as 1,8-diazabicycloundec-7-ene (DBU) or 1,4-diazabicyclo[2.2.2]octane (DABCO) or a C6-10 primary amine, the thiol-Michael addition reaction is fast and the cure time is typically less than 2 hours. Without a strong base catalyst, such as triethylamine, the Michael addition reaction between, for example, a thiol-terminated polythioether and a Michael acceptor is slow providing a pot life, for example, depending on the temperature, of several days to weeks. However, the physical properties of the cured composition are less than desired. The reaction mechanisms for thiol-Michael addition reactions are disclosed by Chan et al., Macromolecules 2010, 43, 6381-6388.
In practice, the foregoing compositions can be provided as two-part compositions in which the thiol-terminated compound and the Michael acceptor are provided as separate components, with the amine catalyst in one or both components, and the two parts are mixed shortly prior to use. For example, if the catalytic amine is a tertiary amine, the amine catalyst may be in one or both components, and if the catalytic amine is a primary or secondary amine, the amine catalyst can only be included in the component containing the thiol-terminated compound. Alternatively, the base catalyst may be provided as a third component, and the component containing the thiol-terminated compound, the component containing the Michael acceptor, and the component containing the base catalyst are mixed shortly before use. However, once the components are mixed, the Michael addition reaction proceeds, and depending at least in part on the temperature and on the type of amine catalyst, the pot life is limited to less than 2 hours. Furthermore, as the composition starts to cure, there is little ability to control the reaction rate to take advantage of the complex chemistries taking place after the sealant is applied to a surface.
Ways to prolong the pot life and to control the curing rate of sulfur-containing polymer compositions employing Michael addition curing chemistries are desired.